The number of unmarried opposite-sex couples in the U.S. has increased dramatically over the past three decades, climbing from 500,000 in 1970 to over 5 million today. More than t .2 million persons ages 50 and over are involved in cohabiting relationships. However, research on cohabitation has essentially ignored older adults. This omission is problematic for at least two reasons. First, from a demographic perspective, the union statuses and histories of today's older adults are more diverse than those of previous cohorts; consequently, as they age into the later years progressively fewer older persons will be married and more will be cohabiting. Second, from a theoretical standpoint, cohabitation likely has a unique meaning and plays a different role in the life course of older (versus younger) adults. That is, the motivations for entering a cohabiting union may differ from those of young adults and thus the union patterns and dynamics of older adults may be distinct. We propose to contribute to the limited research on cohabitation among older adults by using longitudinal data from the 1998-2002 Health and Retirement Study (HRS) to examine transitions into and out of cohabiting unions among adults over age 50. The current investigation is guided by a theoretical framework that incorporates life course and demographic perspectives on aging. We propose to answer the following three questions. Who are older cohabitors and how do they compare to older marrieds and unpartnereds? What factors predict forming a cohabiting union? What factors predict older cohabitors union transitions out of cohabitation (either through marriage or separation)? By documenting the trends and patterns of cohabitation as well as the predictors of union formation and dissolution among older adults, this project will establish the groundwork necessary to motivate a large-scale investigation of these emerging trends and their consequences for a new generation of aging adults.